Abū Bakr Aḥmad ibn Ḥusayn ibn ʿAlī ibn Mūsā al-Khusrawjirdī al-Bayhaqī (Persian: ابو بكر احمد بن حسين بن علي بن موسی خسروجردی بيهقی, 994–1066),[11] also known as Imām al-Bayhaqī, was a Sunni scholar widely known for being the foremost leading hadith master in his age, leading authority in the Shafi'i school, leading authority on the foundation of doctrine, meticulous, a devoted ascetic and one of the notable defenders of the Ash'ari school.
Al-Bayhaqi belongs to the third generation of Ash'ari school and took kalam from two prominent theologians, Ibn Furak and Abu Mansur al-Baghdadi.
In the meantime, al-Bayhaqi had written a lengthy letter to al-Kunduri that has survived, demonstrating the Ash'arites' immunity from heretical accusations and requesting an end to the persecution.
It is known that perpetual fast (Sawm al-Dahr) is a famous practise done by several companions and the Salaf such as Umar, Uthman, Ibn Shihab al-Zuhri, Abu Hanifa, Al-Shafi'i, Al-Tustari, etc.
In these textual based theological works, al-Bayhaqi establishes the Ash'ari school are true Sunnis who are with accordance to the creed of the pious predecessors.
Al Bayhaqi said:[20] The prophet (PBUH) pointed to Abu Musa al-Ash'ari in relation to the verse: (Allah will bring a people whom HE loves and who love Him) (5:54) saying: "They are that man's people," due to the tremendous merit and noble rank attributed by this Hadith to the Imam Abu al-Hassan al-Ash'ari.
For he is part of Abu Musa's people and one of his children who received knowledge and were granted discernment, and he was singled out for strengthening the Sunnah and repressing innovation by producing clear proofs and dispelling doubts.
Therefore, whoever leans towards them in science of foundation of Religion and follows their position in disowning tashbih (likening Allah with His creation) while adhering to the Book and the Sunnah, is one of their number (people).Al-Bayhaqi also had a variety of views stating his understanding of cosmology.
It is as he said, and if al-Bayhaqi had wanted to found a school of Law (madhab) for himself he would have been able to do so, due to the vastness of his sciences and his thorough knowledge of juridical differences (legal matters).
He then assembled his Al-Sunan al-Kubra, a gigantic collection of hadiths that included prophetic traditions and companions opinions to support every point of Shafi'i's substantive law.
Works like Sunan al-Kubra and Al-Sunan al-Wusta championed the body of substantive law of the school and the Shafi'i transmission-based legal methodology.
He was a sublime jurist (faqih jalil), an eminent (kabir) hafiz; an adept jurisprudent (usulu nihrir); abstinent; godly; obedient to Allah (qanit li'llah); firm in supporting the Shafi'i legal school, in terms of methodology (aslan) and derivation of law (fur'ian); and a mountain from the mountains of knowledge...He was the muhaddith of his time (zaman) Shaykh of the Sunnah in his era (waqt).